Many cryogenic systems, including magnetic resonance imaging systems, use a cryogenic refrigeration unit to maintain minimal helium boiloff from a liquid helium bath. A refrigeration unit often may extend into a housing which extends into the chamber containing the liquid helium bath. The refrigerator keeps radiation shields within the housing at a relatively low temperature to minimize the temperature gradient about the liquid helium chamber.
Occasionally, it is necessary to remove the refrigeration unit for repair or cleaning purposes. Since the housing is usually at cryogenic temperatures, to remove the refrigeration unit and expose the housing to an uncontrolled atmosphere would result in a rapid formation of ice within the housing as water vapor comes in contact with the housing walls. This then restricts or prevents the replacement of the removed unit. Such a freeze-up can also cause thermal expansion of joints in the housing. One approach to overcoming the freeze-up problem has been to cover the refrigerator with a tent purged with nitrogen before removal of the unit but such an approach is cumbersome. Hot air guns have also been suggested for removing the ice but have not proven successful.